1. Field of the Invention
The present invention relates generally to systems that use optical fiber and optical fiber components, and particularly to such systems that include circularly polarized waveguide fiber.
2. Technical Background
The optical non-linearities that affect light wave transmission systems fall into two general categories. In the first category are stimulated scattering phenomena, such as stimulated Brillouin scattering and stimulated Raman scattering. These effects are interactions between an optical signal and a phonon in the transmission material. The frequency of the phonon determines the type of scattering that occurs. In the second category, the nonlinear index of refraction gives rise to three effects, self phase modulation (SPM), cross phase modulation (XPM), and four wave mixing (4WM). Based on studies of long-distance, multi-wavelength systems, the second category of nonlinear interactions are most deleterious for wavelength division multiplexed (WDM) systems, especially those having electronic regenerator spacing greater than about 50 km. This second category of non-linear effects is the subject of the present application.
In SPM, the nonlinear index, which depends upon pulse intensity, leads to phase modulation of those pulses above a threshold intensity. The threshold intensity depends upon the material used in the waveguide but is generally of the order of 10 mW. One of the consequences of SPM is that the spectral width of signal pulses gradually increases as they propagate in the fiber. For operation near the zero dispersion wavelength of the waveguide, the spectral broadening of the signal will not degrade system performance. However, if there is sufficient group velocity dispersion, then the spectral broadening from SPM will result in temporal broadening of the pulses. Alternately, in densely spaced WDM systems, cross-talk will occur if the spectral broadening is large enough to cause spectra of a broadened signal to appear in adjacent channels to overlap those channels.
For WDM systems, the intensity variations in one channel can affect the other channels through XPM. For linearly polarized fiber, the XPM coefficient, which indicates the size of the effect, is about twice as large as the SPM coefficient. XPM does depend upon the length of waveguide over which interaction between pulses occurs so that change in spacing (walk-off) between channels due to group-velocity dispersion affects the interaction length and thus the amount of XPM. For sufficiently long systems, the group velocities of various channels will lead to complete walk-through between the channels. Thus, under loss-free conditions, the spectral broadening from XPM is virtually eliminated.
Four wave mixing also arises from the nonlinear index of refraction, but, unlike SPM and XPM, 4WM has a phase matching requirement. For signals at two different wavelengths, the intensity modulation at the beat frequency of the waveguide modulates the refractive index, thus producing a phase modulation at the difference frequency of the two signals. Consequently, in 4WM, side-band frequencies are generated at the original frequencies plus and minus the difference frequencies (the lower frequency side band is called the Stokes frequency, and the higher frequency side band is called the anti-Stokes frequency). The phase-matching requirement means that the index or speed at the two signal wavelengths must coincide with the index or speed of the Stokes and anti-Stokes waves. Therefore, 4WM depends strongly on total dispersion. For high total dispersion, the difference in propagation velocities at the different frequencies is large, and the efficiency of 4WM is poor. In fibers with the zero dispersion wavelength near the signal wavelengths, all waves are nearly coincident in index and speed and the 4WM process can be extremely efficient. In WDM systems, 4WM has two deleterious effects. First is the depletion of power from the signal wavelengths into the mixing products. Second, in systems that have equally spaced signal channels, the Stokes and anti-Stokes frequencies coincide with existing channels causing cross-talk. Also, the mixing products can interfere constructively or destructively with the existing channels, depending on the relative phases of the signals.
In high performance transmission systems, therefore, there is a need for a system configuration, which can include a particular type of optical waveguide fiber, that permits operation close to the zero dispersion wavelength, thus minimizing the linear dispersion penalty, but which still limits the non-linear effects, especially 4WM.